Moving picture coding method and moving picture decoding method

ABSTRACT

A moving picture coding apparatus  1  includes a counter unit  102  which counts the number of pictures following an intra coded picture; and a motion estimation unit  101  which compares respectively only reference pictures which are the intra coded picture or the following pictures, selected from among a reference picture Ref 1 , a reference picture Ref 2  and a reference picture Ref 3  stored in memories  408˜410 , with a picture signal Vin, and determines the reference picture whose inter picture differential value is smallest.

This application is a divisional of application Ser. No. 10/432,872,filed May 28, 2003 now U.S. Pat. No. 7,209,520 which is the NationalStage of International Application No. PCT/JP02/10246, filed Oct. 2,2002.

TECHNICAL FIELD

The present invention relates to a moving picture coding method, amoving picture coding apparatus, a moving picture decoding method and amoving picture decoding apparatus for coding or decoding a movingpicture signal with reference to a plurality of coded pictures ordecoded pictures, and to a recording medium which stores a program forexecuting such coding or decoding in the form of software.

BACKGROUND ART

In the age of multimedia which integrally handles audio, video and otherinformation such as pixel values, existing information media, i.e.,newspapers, magazines, televisions, radios, telephones and other meansthrough which information is conveyed to people, have recently come tobe included in the scope of multimedia. Generally, multimedia refers tosomething that is represented by associating not only characters, butalso graphics, voices, and especially pictures and the like together,but in order to include the aforementioned existing information media inthe scope of multimedia, it appears as a prerequisite to represent suchinformation in digital form.

However, when calculating the amount of information contained in each ofthe aforementioned information media as the amount of digitalinformation, while the amount of information per character is 1˜2 bytes,the amount of information to be required for voice is 64 Kbits or overper second (telephone quality), and 100 Mbits or over per second formoving pictures (current television reception quality), and it is notrealistic for the aforementioned information media to handle such anenormous amount of information as it is in digital form. For example,although video phones are already in the actual use via IntegratedServices Digital Network (ISDN) which offers a transmission speed of 64Kbps˜1.5 Mbps, it is not practical to transmit video information shot bytelevision cameras directly through ISDN.

Against this backdrop, information compression techniques have becomerequired, and moving picture compression techniques compliant with H.261and H.263 standards recommended by ITU-T (InternationalTelecommunication Union-Telecommunication Standardization Sector) areemployed for video phones, for example. Moreover, according toinformation compression techniques compliant with the MPEG-1 standard,it is possible to store picture information in an ordinary music CD(compact disc) together with sound information.

Here, MPEG (Moving Picture Experts Group) is an international standardon digital compression of moving picture signals, and MPEG-1 is astandard for compressing television signal information approximatelyinto one hundredth so that moving picture signals can be transmitted ata rate of 1.5 Mbps. Furthermore, since transmission speed within thescope of the MPEG-1 standard is limited primarily to about 1.5 Mbps,MPEG-2, which was standardized with a view to satisfy requirements forfurther improved picture quality, allows data transmission of movingpicture signals at a rate of 2˜15 Mbps.

Furthermore, MPEG-4 which provides a higher compression ratio has beenstandardized by the working group (ISO/IEC JTC1/SC29/WG11) which wasengaged in the standardization of MPEG-1 and MPEG-2. Not only is itpossible to perform a highly efficient coding at a low bit rate, MPEG-4employs a powerful technique for error resilience which lessens thedegradation of picture quality to be judged from a subjectivestandpoint, even when a transmission channel error occurs. Also, ITU-Thas started work for standardization of H.26L as a next-generationpicture coding method.

MPEG-1, MPEG-2 and MPEG-4 have allowed substantial improvement ofcompression ratio using inter predictive picture coding (hereinafterreferred to as inter picture coding) for coding or decoding adifferential value between a current picture to be coded or decoded anda reference picture (a picture signal of a picture which has been codedor decoded most recently) with reference to the reference picture (See,for example, ISO/IEC 13818-2 “INTERNATIONAL STANDARD Informationtechnology—Generic coding of moving pictures and associated audioinformation: Video”, Dec. 15, 2000, p. 7, Intro. 4.1.1).

In addition, compression of the amount of information can be realized byreducing redundancies in the temporal and spatial directions. In theinter predictive picture coding which aims at reducing the temporalredundancies, a predictive picture is created with reference topreviously coded or decoded pictures (reference pictures) and thedifferential value between the resulting predictive picture and acurrent picture to be coded is coded. Here, a picture is a termrepresenting one sheet of an image, and specifically, a picture means aframe in a progressive image and a frame or a field in an interlaceimage.

As of September, 2001, the proposed H.26L standard allows not onlyreference only to a picture which has been coded or decoded immediatelybefore a current picture to be coded or decoded, but also reference toan arbitrary picture selected, as a reference picture, from a pluralityof pictures which have been coded or decoded prior to the currentpicture.

FIG. 1 shows an illustration of a concept of a conventional movingpicture coding method and a moving picture decoding method. FIG. 1 is anexample in which an arbitrary picture is selected as a reference picturefrom 3 pictures preceding a current picture to be coded or decoded. InFIG. 1, pictures are ordered in display order, and the display time ofthe picture at the far left is earliest. The pictures are also coded inthis order from the left. Therefore, in a bit stream, the pictures arealso ordered in the order of Picture J₁, Picture J₂, Picture J₃ andPicture J₄. When a current picture to be coded or decoded is Picture J₄,it is possible to select one of these Picture J₁, Picture J₂ and PictureJ₃ as a reference picture, and when a current picture to be coded ordecoded is Picture J₅, it is possible to select one of these Picture J₂,Picture J₃ and Picture J₄ as a reference picture.

FIG. 2 is a block diagram showing the structure of a conventional movingpicture coding apparatus.

A moving picture coding apparatus 4 is an apparatus for compressing andcoding an inputted picture signal Vin so as to output a coded picturesignal Str in the form of a bit stream transformed by variable lengthcoding or the like, and is comprised of a motion estimation unit 401, aselection unit 402, a picture signal subtraction unit 403, a coding unit404, a decoding unit 405, an addition unit 406, a selection unit 407 andmemories 408˜410.

The motion estimation unit 401 reads out previously coded referencepictures which are stored in the memories 408˜410 respectively andcompare them with the inputted picture signal Vin so as to determinemotion information MV indicating the reference picture Ref whose interpicture differential value (error energy) is smaller and the pixellocation for making the inter picture differential value smaller. Thereference picture Ref and the pixel location for making the error energysmallest is usually determined, but recently a method has been used fordetermining the motion information MV not just for making the errorenergy smallest but for making the error energy smaller and making thecompression ratio larger. Note that the information of the referencepicture Ref and the pixel location is hereinafter referred to as motioninformation MV collectively. The selection unit 402 outputs thereference picture selected from among a reference picture Ref1, areference picture Ref2 and a reference picture Ref3 which are stored inthe memories 408˜410, based on a reference picture instruction signalRefFrm that is a switching instruction signal. The subtraction unit 403calculates a differential picture signal Dif between a picture signalVin and a reference picture Ref.

The coding unit 404 codes the differential picture signal Dif and themotion information MV that is the information for identifying areference picture. The decoding unit 405 decodes coded data Coded whichhas been coded by the coding unit 404 to obtain a reconstructeddifferential picture signal RecDif. The addition unit 406 adds thereference picture Ref and the reconstructed differential picture signalRecDif. The selection unit 407 outputs the inputted decoded picturesignal Recon to any of the memories 408˜410 as a decoded picture signalRec1, a decoded picture signal Rec2 or a decoded picture signal Rec3, soas to enable the decoded picture signal to be referred to for coding thefollowing pictures.

Next, the operation of the moving picture coding apparatus structured asabove will be explained.

The picture signal Vin is inputted to the picture signal subtractionunit 403 and the motion estimation unit 401. The motion estimation unit401 reads out the reference picture Ref1, the reference picture Ref2 andthe reference picture Ref3 which are previously decoded pictures storedin the memories 408˜410, compares them with the inputted picture signalVin so as to determine a reference picture whose inter picturedifferential value is smallest, and outputs the motion information MVthat is the information for identifying the reference picture and thepixel location to be referred to.

At the same time, the motion estimation unit 401 outputs a referencepicture instruction signal RefFrm that is a switching instruction signalso that the selection unit 402 can select a reference picturecorresponding to the motion information MV and output it as a referencepicture Ref. Note that since a scene change or the like causes a loss ofcorrelation between the pictures, the compression ratio of an intercoded picture could decline more than an intra coded picture (or anintra picture) which can be reconstructed only with a coded picturesignal of a picture to be coded. In this case, the motion estimationunit 401 indicates the intra picture coding by the motion information MVand outputs a reference picture instruction signal RefFrm for outputtinga reference picture Ref4 which always outputs a value 0 as a referencepicture Ref. Note that the value of the reference picture Ref4 does notalways need to be 0 and may be an average value 128 in the case of aluminance signal or an RGB color signal whose value is 0˜255, forexample.

Also, in order to prevent error propagation or enable reproduction tostart from a picture at some midpoint in a coded picture signal, apicture in every predetermined number of pictures needs to be intracoded so as to be reconstructed only with a coded picture signal of apicture to be coded. So, the motion estimation unit 401 can switch intothe intra picture coding forcibly according to the instruction of anintra picture coding instruction signal Reset given from outside.

On the other hand, the subtraction unit 403 calculates the differencebetween this picture signal Vin and the reference picture Ref selectedby the selection unit 402, and outputs the differential picture signalDif to the coding unit 404. Next, the coding unit 404 codes thedifferential picture signal Dif and the motion information MV outputtedfrom the motion estimation unit 401, and outputs the coded picturesignal Str and the coded data Coded. Here, the coded data Coded is datanecessary for reconstructing a picture, and the coded picture signal Stris a bit stream of the coded data Coded transformed by variable lengthcoding or the like.

The decoding unit 405 decodes the coded data Coded and outputs thereconstructed differential picture signal RecDif to the addition unit406. The addition unit 406 adds the reconstructed differential picturesignal RecDif and the reference picture Ref selected by the selectionunit 402, and outputs the decoded picture signal Recon to the selectionunit 407. The selection unit 407 outputs the decoded picture signalRecon to any of the memories 408˜410 as a decoded picture signal Rec1, adecoded picture signal Rec2 or a decoded picture signal Rec3 so that thedecoded picture signal Recon can be referred to as a reference picturefor coding the following pictures. In this example, the selection unit407 switches the memories so that the picture which has been stored inany of these memories at the earliest time is overwritten by a newdecoded picture signal Recon.

FIG. 3 is a block diagram showing the structure of a conventional movingpicture decoding apparatus.

A moving picture decoding apparatus 5 is an apparatus for decoding acoded picture signal Str which has been coded by the moving picturecoding apparatus 4.

A decoding unit 501 decodes the inputted coded picture signal Str andoutputs a reconstructed differential picture signal RecDif and motioninformation MV. A motion reconstruction unit 502 decodes the motioninformation MV and outputs a reference picture instruction signalRefFrm. Operations of a selection unit 503, a selection unit 505 andmemories 506˜508 are same as those of the selection unit 402, theselection unit 407 and the memories 408˜410 of the moving picture codingapparatus 4 as shown in FIG. 2. The addition unit 504 adds thereconstructed differential picture signal RecDif and the referencepicture Ref to output a decoded picture signal Vout (which correspondsto a decoded picture signal Recon in FIG. 2).

Note that in the above-mentioned moving picture coding apparatus 4 andthe moving picture decoding apparatus 5, motion compensation units notshown in the figures are provided on the output sides of the selectionunit 402 and the selection unit 503, respectively, and perform motioncompensation for generating pixel values with pixel location precisionin decimal degree which are pixel values with ½ pixel location precisionor the like for interpolating the pixel values of the reference pictureoutputted from the memory.

By the way, in the above-mentioned conventional moving picture codingapparatus and moving picture decoding apparatus, it is not distinguishedat all whether the reference picture is an intra coded picture or one ofinter coded pictures following the intra coded picture. For example, inthe illustration of FIG. 1 explaining the concept of the conventionalmoving picture coding method and moving picture decoding method, PictureJ₂ is an intra coded picture and Picture J₁, Picture J₃, Picture J₄ andPicture J₅ are inter coded pictures, but Picture J₁ can be referred toas a reference picture for Picture J₄. If Picture J₄ refers to PictureJ₁ as a reference picture, it means that Picture J₄ refers to, as areference picture, the picture preceding the intra coded Picture J₂.

However, when starting reproduction from a picture at some midpoint, forexample, when starting decoding and reproduction from the intra codedPicture J₂ at a midpoint in a coded picture signal, the decoding ofPicture J₄ requires reference to the decoded Picture J₁. Therefore, aproblem occurs that the pictures following Picture J₄ cannot becorrectly decoded.

Also, for example, if a stream error occurs at a midpoint in a codedpicture signal and Picture J₁ cannot be correctly decoded due to theerror, a problem occurs that the pictures following Picture J₄ cannot becorrectly decoded because the decoding of Picture J₄ requires referenceto Picture J₁, although the intra coded Picture J₂ can be correctlydecoded.

And so, the present invention has been conceived in view of theabove-mentioned circumstances, and aims at providing a moving picturecoding method, a moving picture decoding method and the like for makingit possible to start reproduction from an intra coded picture at amidpoint in a coded picture signal and thus reproduce the picturesfollowing the intra coded picture without error even if a stream errormay occur.

DISCLOSURE OF INVENTION

The moving picture coding method according to the present invention is amethod for coding a moving picture on a picture by picture basis, themoving picture coding method comprising: a determining step fordetermining, as a reference picture, one of an intra coded picture andpictures which have been coded following the intra coded picture, fromamong a plurality of pictures stored in a memory; a coding step forreading out the determined reference picture from the memory,calculating a differential picture signal that is a difference betweenthe read-out reference picture and a picture to be coded, and coding thecalculated differential picture signal; and a storing step for decodingthe coded differential picture signal, adding the decoded differentialpicture signal with a picture signal of the reference picture, andstoring a resulting picture in the memory.

Also, the moving picture coding method according to the presentinvention is a method for coding a moving picture on a picture bypicture basis, the moving picture coding method comprising: a specifyingstep for specifying an intra coded picture of a plurality of picturesstored in a memory as a picture which is an anchor for restricting acandidate for a reference picture; and a coding step for coding a signalindicating the picture which is an anchor for restricting a candidatefor a reference picture so that a picture to be coded following thepicture specified in the specifying step refers only to said specifiedpicture or pictures which have been coded following said specifiedpicture.

In addition, the moving picture coding method according to the presentinvention is a method for coding a moving picture on a picture bypicture basis, the moving picture coding method comprising: a specifyingstep for specifying an intra coded picture of a plurality of picturesstored in a memory as a picture which is an anchor for restricting acandidate for a reference picture; a step for deleting a picture whichhas been coded prior to the picture specified in the specifying step;and a coding step for coding a signal indicating that the picture whichhas been coded prior to the picture specified in the specifying step isdeleted so that a picture to be coded following said specified picturerefers only to said specified picture or pictures which have been codedfollowing said specified picture.

Further, the moving picture decoding method according to the presentinvention is a method for decoding a moving picture on a picture bypicture basis, the moving picture decoding method comprising: a decodingstep for decoding an inputted coded picture signal; a determining stepfor determining, as a reference picture, one of an intra decoded pictureand pictures which have been decoded following the intra decodedpicture, from among a plurality of pictures stored in a memory; and astoring step for reading out the determined reference picture from thememory, adding a picture signal of the read-out reference picture with adecoded differential picture signal of a picture to be decoded, andoutputting a resulting picture to the outside and storing the resultingpicture in the memory.

Also, the moving picture decoding method according to the presentinvention is a method for decoding a moving picture on a picture bypicture basis, the moving picture decoding method comprising: a decodingstep for decoding a signal indicating a picture which is an anchor forrestricting a candidate for a reference picture which is to be referredto for decoding a picture to be decoded; and a determining step fordetermining, as a reference picture, only the picture which is an anchorand intra decoded or one of pictures which have been decoded followingsaid picture, when decoding a picture following said picture.

In addition, the moving picture decoding method according to the presentinvention is a method for decoding a moving picture on a picture bypicture basis, the moving picture decoding method comprising: a decodingstep for decoding a signal indicating a picture which is an anchor forrestricting a candidate for a reference picture which is referred to fordecoding a picture to be decoded; a step for deleting a picture whichhas been decoded prior to the picture which is an anchor and intradecoded; and a determining step for determining, as a reference picture,only the picture which is an anchor and intra decoded or one of pictureswhich have been decoded following said picture, when decoding a picturefollowing said picture.

The moving picture coding apparatus according to the present inventionis a moving picture coding apparatus for coding a moving picture on apicture by picture basis, the moving picture coding apparatuscomprising: a determination unit operable to determine, as a referencepicture, one of an intra coded picture and pictures which have beencoded following the intra coded picture, from among a plurality ofpictures stored in a memory; a coding unit operable to read out thedetermined reference picture from the memory, calculate a differentialpicture signal that is a difference between the read-out referencepicture and a picture to be coded, and code the calculated differentialpicture signal; and a storage unit operable to decode the codeddifferential picture signal, add the decoded differential picture signalwith a picture signal of the reference picture, and store a resultingpicture in the memory.

Also, the moving picture decoding apparatus according to the presentinvention is a moving picture decoding apparatus for decoding a movingpicture on a picture by picture basis, the moving picture decodingapparatus comprising: a decoding unit operable to decode an inputtedcoded picture signal; a determination unit operable to determine, as areference picture, one of an intra decoded picture and pictures whichhave been decoded following the intra decoded picture, from among aplurality of pictures stored in a memory; and a storage unit operable toread out the determined reference picture from the memory, add a picturesignal of the read-out reference picture with a decoded differentialpicture signal of a picture to be decoded, and output a resultingpicture to the outside and store the resulting picture in the memory.

Further, the present invention can be realized as a program for causinga computer to execute the steps in the above-mentioned moving picturecoding method and the moving picture decoding method, or as stream datawhich is coded by the above-mentioned moving picture coding method, soas to be distributed via a recording medium such as a CD-ROM or atransmission medium such as a communication network.

For example, it is assumed that the pictures are coded in the order ofPicture J₁, Picture J₂, Picture J₃, Picture J₄ and Picture J₅, PictureJ₂ is an intra coded picture and Picture J₁, Picture J₃, Picture J₄ andPicture J₅ are inter coded pictures. Specifically, their order in a bitstream is Picture J₁, Picture J₂, Picture J₃, Picture J₄ and Picture J₅.

In this case, in the moving picture coding method and the moving picturedecoding method according to the present invention, the followingpictures can be selected as reference pictures for inter picture codingand decoding: Picture J₂, Picture J₃ and Picture J₄ can be referred tofor coding Picture J₅, for example; and Picture J₂ and Picture J₃ canonly be referred to for coding Picture J₄ because Picture J₁ precedingthe intra coded Picture J₂ is disabled for reference.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an illustration of a concept of a conventional moving picturecoding method and moving picture decoding method.

FIG. 2 is a block diagram showing a structure of a conventional movingpicture coding apparatus.

FIG. 3 is a block diagram showing a structure of a conventional movingpicture decoding apparatus.

FIG. 4 is a block diagram showing a structure of a moving picture codingapparatus of a first embodiment according to the present invention.

FIG. 5 is a flowchart showing operation of a motion estimation unit inthe above embodiment.

FIG. 6 is an illustration showing pictures which can be selected asreference pictures for coding pictures in the above embodiment.

FIG. 7 is a block diagram showing another example of the moving picturecoding apparatus of the first embodiment according to the presentinvention.

FIG. 8 is a block diagram showing a structure of a moving picturedecoding apparatus of the first embodiment according to the presentinvention.

FIG. 9 is a flowchart showing operation of a motion reconstruction unitin the above embodiment.

FIG. 10 is an illustration explaining selection of reference picturesused for decoding by methods 1˜3 in the above embodiment, and A is anillustration of reference pictures used for decoding, and B is anillustration showing picture location relation.

FIG. 11 is a block diagram showing a structure of a moving picturedecoding apparatus of a second embodiment according to the presentinvention.

FIG. 12 is a flowchart showing operation of a motion reconstruction unitin the above embodiment.

FIG. 13 is an illustration of a recording medium for storing a programfor realizing the moving picture coding method and the moving picturedecoding method of the above-mentioned first embodiment and the secondembodiment by a computer system. A is an illustration showing an exampleof a physical format of a flexible disk that is the recording mediummain body, B is an illustration showing the front view of the appearanceof the flexible disk, the cross-sectional view thereof and the flexibledisk, and C is an illustration showing the structure for recording andreproducing the above program on the flexible disk FD.

FIG. 14 is a block diagram showing an overall configuration of a contentproviding system which realizes content distribution services.

FIG. 15 is a diagram showing an example of a mobile phone.

FIG. 16 is a block diagram showing an internal structure of the mobilephone.

FIG. 17 is a block diagram showing an overall configuration of a digitalbroadcasting system.

BEST MODE FOR CARRYING OUT THE INVENTION

The embodiments of the present invention will be explained below withreference to FIG. 4˜FIG. 17.

First Embodiment

FIG. 4 is a block diagram showing a structure of a moving picture codingapparatus of a first embodiment according to the present invention. Notethat the same signs are assigned to the devices which operate in thesame manner as the units of the conventional moving picture codingapparatus 4 as shown in FIG. 2.

A moving picture coding apparatus 1 is an apparatus that compresses andcodes an inputted picture signal Vin, transforms it into a bit stream byvariable length coding or the like and outputs the resulting codedpicture signal Str, and is comprised of a motion estimation unit 101,the selection unit 402, the picture signal subtraction unit 403, thecoding unit 404, the decoding unit 405, the addition unit 406, theselection unit 407, the memories 408˜410 and a counter unit 102.

When receiving the instruction of intra picture coding by an intrapicture coding instruction signal Reset inputted from outside, thecounter unit 102 start counting the number of pictures following anintra coded picture, and notifies the motion estimation unit 101 of thecounting result as the number of potential reference pictures (pictureswhich can be referred to) Num.

The motion estimation unit 101 compares a picture signal Vin with onlyreference pictures which have been previously coded and decoded intracoded picture and the following pictures from among the referencepicture Ref1, the reference picture Ref2 and the reference picture Ref3which are stored in the memories 408˜410, respectively, based on thenumber of potential reference pictures, and determines motioninformation MV indicating the reference picture Ref whose inter picturedifferential value (error energy) is smaller and the pixel location formaking the inter picture differential value smaller. Note that thereference picture Ref and the pixel location for making the error energysmallest is usually determined, but the motion information MV may bedetermined for not just for making the error energy smallest, but formaking the error energy smaller and making the compression ratio larger.A reference picture instruction signal RefFrm is outputted to theselection unit 402 in order to indicate the reference picture Ref.

The selection unit 402 outputs the reference picture Ref selected fromamong the reference picture Ref1, the reference picture Ref2 and thereference picture Ref3 which are stored in the memories 408˜410, basedon a reference picture instruction signal RefFrm that is a switchinginstruction signal.

The subtraction unit 403 calculates a differential picture signal Difbetween the picture signal Vin and the reference picture Ref. The codingunit 404 codes the differential picture signal Dif and the motioninformation MV that is the information for identifying a referencepicture, and outputs a coded picture signal Str and coded data Coded.Here, the coded data Coded is the data necessary for reconstructing apicture (data obtained by coding the motion information MV and thedifferential picture signal Dif), and the coded picture signal Str is abit stream obtained by further transforming the coded data Coded byvariable length coding or the like.

The decoding unit 405 decodes coded data Coded to obtain a reconstructeddifferential picture signal RecDif. The addition unit 406 adds thereference picture Ref and the reconstructed differential picture signalRecDif. The selection unit 407 outputs the inputted decoded picturesignal Recon to any of the memories 408˜410 as a decoded picture signalRec1, a decoded picture signal Rec2 or a decoded picture signal Rec3, sothat the decoded picture signal Recon can be referred to as a referencepicture for coding the following pictures. In the present embodiment,the selection unit 407 switches the memories so that the decoded picturesignal stored in the memory at the earliest time is overwritten by a newdecoded picture signal Recon.

Next, the operation of the moving picture coding apparatus structured asabove will be explained.

FIG. 5 is a flowchart showing the operation of the motion estimationunit 101.

The picture signal Vin is inputted to the picture signal subtractionunit 403 and the motion estimation unit 101.

Upon receipt of the picture signal Vin, the motion estimation unit 101reads out the reference picture Ref1 stored in the memory 408 (Step S1).Next, the motion estimation unit 101 judges whether or not thisreference picture Ref1 is an intra coded picture or one of the followingpictures. Specifically, it judges using an equation (A), for example,based on the picture number specific to each picture and the number ofpotential reference pictures which is notified from the counter unit 102(Step S2). Here, the picture number is an ID number assigned to a codedpicture and has the following feature.

Specifically, the picture number of a picture signal Vin is larger by 1than the picture number of a picture which has been stored most recentlyamong the pictures which have been stored in the memories 408˜410 andare to be reference pictures.

$\begin{matrix}{{{Picture}\mspace{14mu}{number}\mspace{14mu}{of}\mspace{14mu} a\mspace{14mu}{reference}\mspace{14mu}{picture}} \geqq {{{Picture}\mspace{14mu}{number}\mspace{14mu}{of}\mspace{14mu}{Picture}\mspace{14mu}{signal}\mspace{14mu}{Vin}} - {{Number}\mspace{14mu}{of}\mspace{14mu}{potential}\mspace{14mu}{reference}\mspace{14mu}{pictures}\mspace{14mu}{Num}}}} & (A)\end{matrix}$

As a result, when the above equation (A) is satisfied, the motionestimation unit 101 calculates the differential value between thepicture signal Vin and the reference picture Ref1 because the referencepicture Ref1 is an intra coded picture or the following picture (StepS3), while it does not calculate the differential value when theequation (A) is not satisfied.

Next, the motion estimation unit 101 performs the processing for theabove-mentioned reference picture Ref1 respectively for the referencepicture Ref2 stored in the memory 409 and the reference picture Ref3stored in the memory 410 in the same manner (Step S1˜Step S3).

Next, the motion estimation unit 101 determines a reference picturewhose inter picture differential value is smallest from among thereference pictures whose differential values have been calculated asabove (Step S4). Then, the motion estimation unit 101 outputs theinformation for identifying the determined reference picture as motioninformation MV, and, at the same time, outputs a reference pictureinstruction signal RefFrm that is a switching instruction signal so thatthe selection unit 402 can select the determined reference picture tooutput it as a reference picture Ref (Step S5).

Note that a reference picture whose inter picture differential value issmallest may be determined in Step S4 after the reference pictures whosedifferential values have been calculated in Step S3 so as to repeat StepS1, Step S2, Step S3 and Step S4 for the memories 408˜410 respectively.

On the other hand, upon receipt of the picture signal Vin, thesubtraction unit 403 calculates a difference between this picture signalVin and the reference picture Ref selected by the selection unit 402,and outputs a differential picture signal Dif to the coding unit 404.Next, the coding unit 404 codes the differential picture signal Dif andthe motion information MV outputted from the motion estimation unit 101to output a coded picture signal Str and coded data Coded.

The decoding unit 405 decodes the coded data Coded and outputs thereconstructed differential picture signal RecDif to the addition unit406. The addition unit 406 adds the reconstructed differential picturesignal RecDif and the reference picture Ref selected by the selectionunit 402, and outputs the decoded picture signal Recon to the selectionunit 407. The selection unit 407 outputs the decoded picture signalRecon to any of the memories 408˜410 as a decoded picture signal Rec1, adecoded picture signal Rec2 or a decoded picture signal Rec3 so as toenable the decoded picture signal Recon to be referred to for coding thefollowing pictures.

FIG. 6 is an illustration showing pictures which can be selected asreference pictures for coding pictures. As is the case of FIG. 1,Picture J₂ is an intra coded picture, and Picture J₁, Picture J₃,Picture J₄ and Picture J₅ are inter coded pictures. The coding order ina bit stream is Picture J₁, Picture J₂, Picture J₃, Picture J₄ andPicture J₅.

As mentioned above, when inter picture coding is performed under theoperation of the moving picture coding apparatus 1, for example, whenPicture J₅ is coded as shown in FIG. 6, Picture J₂, Picture J₃ andPicture J₄ can be selected as reference pictures. Also, when Picture J4is coded, only Picture J₂ and Picture J₃ can be referred to becausePicture J₁ preceding the intra coded Picture J₂ is disabled for coding.

As described above, since pictures are coded using only an intra codedpicture and pictures which have been coded following the intra codedpicture as reference pictures, reproduction can be started from theintra coded picture at some midpoint of a coded picture signal. Also,even if an error occurs in a stream, a coded picture signal Str, thatis, a stream which can be reproduced without error following the intracoded picture, can be generated.

In order to start reproduction at some midpoint of a picture signalrecorded on a DVD or a hard disk, a mechanism for the midpointreproduction is required, so Reset is instructed at the picture fromwhich reproduction is to be started. This Reset instruction may bedetermined and made by an operator when coding pictures, or at everypredetermined picture cycle or time lapse.

Note that each picture has the information on whether the picture isintra coded or inter coded as auxiliary information. So, a signal,indicating that the intra coded picture which has been explained in theabove first embodiment is specified by an intra picture codinginstruction signal Reset, may also be coded (FIG. 7). Specifically, ifthis is the intra coded picture specified by this intra picture codinginstruction signal Reset, it may be designed so that pictures stored ina memory prior to this specified picture are not referred to.

Or, it means here that pictures stored in a memory prior to an intracoded picture are not referred to, with the intra coded picture set asthe anchor picture, but the pictures stored in the memory prior to theintra coded picture may be deleted from the memory, with the intra codedpicture set as the anchor picture.

In addition, an intra picture coding instruction signal Reset, whichindicates that pictures stored in a memory prior to an intra codedpicture are not referred to, is coded here, but in order to delete thepictures stored prior to the intra coded picture from the memory, acontrol command other than this intra picture coding instruction signalReset may be coded.

FIG. 8 is a block diagram showing the structure of a moving picturedecoding apparatus of the first embodiment according to the presentinvention. Note that the same signs are assigned to the units whichoperate in the same manner as the units in the conventional movingpicture decoding apparatus 5 as shown in FIG. 3.

The moving picture decoding apparatus 2 is an apparatus for decoding acoded picture signal Str which is coded by the moving picture codingapparatus as shown in the above first embodiment, and is comprised of adecoding unit 201, a motion reconstruction unit 202, the selection unit503, the addition unit 504, the selection unit 505, the memories506˜508, and a counter unit 203.

The decoding unit 201 decodes the inputted coded picture signal Str, andoutputs a reconstructed differential picture signal RecDif, motioninformation MV and an intra picture coding instruction signal Reset.Upon receipt of the instruction of intra picture coding by the intrapicture coding instruction signal Reset, the counter unit 203 startscounting the number of pictures following an intra coded picture, andnotifies the motion reconstruction unit 202 of the result as the numberof potential reference pictures Num.

The motion reconstruction unit 202 decodes the motion information MV,judges based on the number of potential reference pictures whether areference picture indicated by this motion information MV is a picturewhich can be referred to or not, and determines the reference picture.

The selection unit 503 selects one picture from among the referencepicture Ref1, the reference picture Ref2 and the reference picture Ref3stored in the memories 506˜508 based on a reference picture instructionsignal RefFrm that is a switching instruction signal, and outputs thepicture as a reference picture Ref. The addition unit 504 adds thereconstructed differential picture signal RecDif and the referencepicture Ref to output a decoded picture signal Vout1 (which correspondsto a decoded picture signal Recon in FIG. 4).

The selection unit 505 outputs the inputted decoded picture signal Vout1to any of the memories 506˜508 as the decoded picture signal Rec1, thedecoded picture signal Rec2 or the decoded picture signal Rec3 in orderto enable the decoded picture signal Vout1 to be referred to as areference picture when decoding the following pictures. In the presentembodiment, the selection unit 505 switches the memories so that apicture which has been stored in any of these memories at the earliesttime is overwritten by a new decoded picture signal Recon.

Next, the operation of the moving picture decoding apparatus structuredas mentioned above will be explained.

FIG. 9 is a flowchart showing the operation of the motion reconstructionunit 202.

The coded picture signal Str is inputted to the decoding unit 201. Thedecoding unit 201 decodes the inputted coded picture signal Str andoutputs a reconstructed differential picture signal RecDif and motioninformation MV. Also, when the inputted coded picture signal Str is anintra coded picture, the decoding unit 201 outputs an intra picturecoding instruction signal Reset.

Upon receipt of the intra picture coding instruction signal Reset fromthe decoding unit 201, the counter unit 203 starts calculating thenumber of pictures following the intra coded picture, and notifies themotion reconstruction unit 202 of the result as the number of potentialreference pictures Num.

Upon receipt of the motion information MV and the number of potentialreference pictures Num, the motion reconstruction unit 202 identifiesthe reference picture which has been referred to for coding based on themotion information MV that is the information for identifying areference picture (Step S11). Next, the motion reconstruction unit 202judges whether or not this reference picture is an intra decoded pictureor one of the following pictures. Specifically, it judges using anequation (B), for example, based on the picture number specific to eachpicture and the number of potential reference pictures which is notifiedfrom the counter unit 203 (Step S12).Picture number of a reference picture≧Picture number of coded picturesignal Str−Number of potential reference pictures Num  (B)

As a result, since the reference picture is an intra decoded picture orone of the following pictures when the above equation (B) is satisfied,the motion reconstruction unit 202 selects the reference pictureidentified based on the motion information MV as a reference pictureused for decoding (Step S13).

On the other hand, when the equation (B) is not be satisfied, thereference picture is an intra decoded picture. Basically, pictures musthave been coded with reference only to an intra coded picture or thefollowing pictures. Therefore, the reference picture necessary fordecoding which has been identified based on the motion information MVshould be an intra decoded picture or one of pictures which have beendecoded following the intra decoded picture and obtained based on thenumber of potential reference pictures Num. However, there is apossibility that a picture which has been decoded prior to the intradecoded picture is referred to, that is, the above equation (B) is notbe satisfied, due to a transmission error or the like. So, the motionreconstruction unit 202 selects a reference picture used for decodingaccording to any of the following methods 1˜3 for predetermining thereference picture in the case when the equation (B) is not be satisfied(Step S14).

FIG. 10 is an illustration explaining selection of a reference pictureused for decoding by methods 1˜3, and A is an illustration of areference picture used for decoding, and B is an illustration showingpicture location relation. This example shows the decoding of PictureP₄, and Picture P₂ is an intra decoded picture and the other Picture P₁,Picture P₃ and Picture P₄ are inter decoded pictures. In both thedisplay order and decoding start (location in a stream) order, thesepictures are ordered, from earliest to latest, in the order of PictureP₁, Picture P₂, Picture P₃ and Picture P₄.

(Method 1)

A reference picture (Picture P₁) identified based on the motioninformation MV is selected as a reference picture to be used fordecoding as it is. In this case, even if a picture preceding an intracoded picture has been referred to for coding due to error, the picturecan be properly decoded if only Picture P₁ has been decoded properly.

(Method 2)

An intra decoded picture (Picture P₂) is selected as a reference pictureto be used for decoding. In this case, since the intra decoded pictureis a picture which has been decoded earliest as a reference pictureamong potential reference pictures, the intra decoded picture has thestrongest correlation with a picture preceding itself among thepotential reference pictures, and thus there is a high possibility thatthe picture quality is not degraded even if the reference picture usedfor coding is a picture which has been decoded prior to the intradecoded picture.

(Method 3)

A picture which has been decoded most recently (Picture P₃) is selectedas a reference picture to be used for decoding. Generally, picturesignals have stronger correlation with each other if the time intervalbetween them is shorter, and, as a result, there is a very highpossibility that the picture which has been decoded most recently is tobe a reference picture. Therefore, the most recently decoded picturewith the strongest correlation is most likely to be a proper referencepicture if a wrong reference picture is identified based on the motioninformation MV, and thus there is a high possibility that the picturequality is not degraded.

Note that any one of these methods 1˜3 may be used, or a combination oftwo or more methods may be used. As an example of such a combination,when in the method 1, the reference picture (Picture P₁) identifiedbased on the motion information MV cannot be selected as a referencepicture to be used for decoding as it is, default processing may beperformed for selecting the picture decoded most recently, as shown inthe method 3, as a reference picture to be used for decoding, forexample.

Next, the motion reconstruction unit 202 reads out in sequence thereference picture Ref1, reference picture Ref2 and reference pictureRef3 stored in the memories 506˜508, and judges whether they are thereference pictures selected as mentioned above or not (Step S15 in FIG.9). In other words, it identifies the memory storing the selectedreference picture among the memories 506˜508. Then, the motionreconstruction unit 202 outputs a reference picture instruction signalRefFrm that is a switching instruction signal so that the selection unit503 can select for outputting the identified reference picture Ref1,reference picture Ref2 or reference picture Ref3 as a reference pictureRef (Step S16 in FIG. 9).

The addition unit 504 adds the reference picture Ref and thereconstructed differential picture signal RecDif outputted from thedecoding unit 201, and outputs a decoded picture signal Vout1 (whichcorresponds to a decoded picture signal Recon in FIG. 4) to theselection unit 505. The selection unit 505 outputs the inputted decodedpicture signal Vout1 to any of the memories 506˜508 as a decoded picturesignal Rec1, decoded picture signal Rec2 or decoded picture signal Rec3,so as to enable the decoded picture signal Vout1 to be referred to as areference picture when decoding the following pictures.

As described above, a coded picture signal Str which has been coded withreference only to an intra decoded picture or pictures following theintra decoded picture as reference pictures can be properly decoded toobtain a decoded picture signal Vout1, and thus reproduction can bestarted from the intra coded picture at some midpoint of a coded picturesignal. Also, even if an error occurs in a stream, pictures followingthe intra coded picture can be reproduced without error.

Note that in the present embodiment, the methods 1˜3 are predeterminedfor selecting a reference picture to be used by the motionreconstruction unit 202 for decoding, but the present invention is notlimited to these methods. These three methods or two of them may bedynamically switched according to the situation, in such a way that, forexample, the method 3 is used when the picture number identified basedon motion information MV is quite different from the potential referencepicture numbers, the method 1 is used when the picture identified basedon the motion information MV is stored in a memory and can be referredto, and the method 2 is used in other cases.

Also, each picture has the information on whether the picture is to beintra decoded or inter decoded as auxiliary information. So, the codedpicture signal Str, including a signal indicating that the intra codedpicture which has been explained in the above first embodiment isspecified by an intra picture coding instruction signal Reset, may bereceived, and that signal indicating the picture specified by the intrapicture coding instruction signal Reset may be decoded. Specifically, ifa picture is an intra coded picture specified by this intra picturecoding instruction signal Reset, it may be designed so that picturesstored in a memory prior to this specified picture are not referred to.

Or, it means here that pictures stored in a memory prior to an intracoded picture are not referred to, with the intra decoded picture set asthe anchor picture, but pictures stored in a memory prior to the intradecoded picture may be deleted from the memory, with the intra decodedpicture set as the anchor picture.

In addition, in order to delete the pictures stored prior to the intracoded picture from the memory, it may be designed so as to receive acoded picture signal Str including a control command other than thisintra picture coding instruction signal Reset, decode this controlcommand, judge whether or not this control command is received alongwith the signal indicating the picture specified by the intra picturecoding instruction signal Reset, and perform error control.

Second Embodiment

Next, a moving picture decoding apparatus of a second embodiment of thepresent invention will be explained.

FIG. 11 is a block diagram showing the structure of the moving picturedecoding apparatus of the second embodiment according to the presentinvention. Note that the same signs are assigned to the units whichoperate in the same manner as those of the moving picture decodingapparatus 2 as shown in FIG. 8

A moving picture decoding apparatus 3 includes a selection unit 301 inaddition to the units of the moving picture decoding apparatus 2 asshown in FIG. 8, and it is different from the first embodiment in itsoperation in the case where a reference picture identified by motioninformation MV is a picture which has been decoded prior to an intradecoded picture due to transmission error or the like.

The selection unit 301 outputs either a decoded picture signal Reconoutputted from the addition unit 504 or a reference picture Refoutputted from the selection unit 503 as a decoded picture signal Vout2,based on an error notice signal Err notified from a motionreconstruction unit 302.

Next, the operation of the moving picture decoding apparatus structuredas mentioned above will be explained, but the explanation of theportions same as the above first embodiment will be omitted.

FIG. 12 is a flowchart showing the operation of the motionreconstruction unit 302.

The operation in Steps S21˜S22 as shown in FIG. 12 is same as theoperation in Steps S11˜S12 as shown in FIG. 9. Next, as a result ofjudging whether the identified reference picture is an intra decodedpicture or one of the following pictures or not, if the above equation(B) is not satisfied, that is, the reference picture is a picture whichhas been decoded prior to the intra decoded picture, the motionreconstruction unit 302 sends an error notice signal Err to theselection unit 301 to notify it of an error. The motion reconstructionunit 302 also selects the picture decoded most recently as a referencepicture and a decoded picture signal (Step S23). In other words, whenreceiving the error notice signal Err, the selection unit 301 outputs areference picture Ref as a decoded picture signal Vout2, so selectingthe picture decoded most recently as a reference picture means selectingthe picture decoded most recently as a decoded picture signal.

On the other hand, when the above equation (B) is satisfied as a resultof the above judgment, the reference picture is an intra decoded pictureor one of the following pictures, so the motion reconstruction unit 302selects, in the same manner as the above first embodiment, a referencepicture identified based on the motion information MV as a referencepicture used for decoding (Step S24).

The operation in the next Steps S25˜S26 as shown in FIG. 12 is same asthe operation in the Steps S15˜S16 as shown in FIG. 9.

Next, the addition unit 504 adds the reference picture Ref and thereconstructed differential picture signal RecDif outputted from thedecoding unit 201 to output a decoded picture signal Recon. When theerror notice signal Err is not notified from the motion reconstructionunit 302, the selection unit 301 outputs the decoded picture signalRecon outputted from the addition unit 504 as a decoded picture signalVout2, and when the error notice signal Err is notified, it outputs thereference picture Ref outputted from the selection unit 503 as a decodedpicture signal vout2. In sum, when the error notice signal Err isnotified, it outputs the picture decoded most recently as it is as adecoded picture signal Vout2.

The selection unit 505 outputs the inputted decoded picture signal Vout2to any of the memories 506˜508 as a decoded picture signal Rec1, adecoded picture signal Rec2 or a decoded picture signal Rec3 so as toenable the decoded picture signal Vout2 to be referred to as a referencepicture when decoding the following pictures.

As described above, even if a picture preceding an intra decoded pictureis referred to due to a transmission error or the like, a referencepicture Ref that is the picture decoded most recently with the strongestpixel correlation is outputted as a decoded picture signal Vout2, so theinfluence on picture degradation due to such an error can be minimized.

Note that in each of the above embodiments, the maximum of 3 codedpictures can be referred to because the number of memories is 3, but thepresent invention is not limited to that number, but coding and decodingcan be performed with reference to more coded pictures with the increaseof the number of memories.

Also, in each of the above embodiments, motion compensation units notshown in the figures are respectively provided on the output sides ofthe selection unit 402 and the selection unit 503 of the moving picturecoding apparatus 1 and the moving picture decoding apparatuses 2 and 3,for performing motion compensation, that is, compensation of pixelmotion between pictures.

In addition, in each of the above embodiments, reference pictures arelimited for inter picture coding, so that any pictures which have beencoded prior to an intra coded picture, if any, are not referred to asreference pictures, but the present invention is not limited to that.For example, it is also possible not to limit reference pictures so thatany pictures decoded prior to an intra coded picture are not referred toas reference pictures. In other words, it is possible to provide anothertype of an intra coded picture so that the pictures coded prior to theintra coded picture can be referred to as reference pictures, as is thecase of a traditional method, and use both types of intra coded picturesaccording to need. In this case, the information for distinguishing twotypes of intra coded pictures can be stored in header information or thelike in a coded picture signal Str, for example.

Further, in each of the above embodiments, the counter unit 102 or 203may count in picture display order, not in picture coding or decodingorder.

Third Embodiment

In addition, if a program for realizing the structure of the picturecoding method or the picture decoding method as shown in each of theabove-mentioned embodiments is recorded on a storage medium such as aflexible disk, it becomes possible to perform the processing as shown ineach of the above embodiments easily in an independent computer system.

FIG. 13 is an illustration showing the implementation by a computersystem using a flexible disk which stores the picture coding method orthe picture decoding method in the above-mentioned first and secondembodiments.

FIG. 13B shows the front view and the cross-sectional view of theappearance of a flexible disk, and the flexible disk, and FIG. 13A showsan example of a physical format of a flexible disk as a recording mediumitself. A flexible disk FD is contained in a case F, a plurality oftracks Tr are formed concentrically on the surface of the disk in theradius direction from the periphery, and each track is divided into 16sectors Se in the angular direction. Therefore, as for the flexible diskstoring the above-mentioned program, the picture coding method and thepicture decoding method as the above program are recorded in an areaallocated for it on the flexible disk FD.

FIG. 13C shows the structure for recording and reproducing the programon and from the flexible disk FD. For recording the program on theflexible disk FD, the computer system Cs writes the picture codingmethod or the picture decoding method as the program on the flexibledisk FD via a flexible disk drive FDD. For constructing the abovepicture coding method and the picture decoding method in the computersystem by the program recorded on the flexible disk, the program is readout from the flexible disk FD via the flexible disk drive FDD andtransferred to the computer system.

Note that the above explanation is made on the assumption that arecording medium is a flexible disk, but the same processing can also beperformed using an optical disk. In addition, the recording medium isnot limited to these, but any other mediums such as an IC card and a ROMcassette can be used in the same manner if a program can be recorded onthem.

Further, the applications of the moving picture coding method and themoving picture decoding methods as shown in the above embodiments and asystem using them will be explained here.

FIG. 14 is a block diagram showing the overall configuration of acontent providing system ex100 for realizing content distributionservice. The area for providing communication service is divided intocells of desired size, and base stations ex107˜ex110 which are fixedwireless stations are placed in respective cells.

This content providing system ex100 is connected to various devices suchas a computer ex111, a PDA (personal digital assistant) ex112, a cameraex113, a mobile phone ex114 and a camera-equipped mobile phone ex115 viathe Internet ex101, an Internet service provider ex102, a telephonenetwork ex104 and base stations ex107˜ex110, for example.

However, the content providing system ex100 is not limited to thecombination as shown in FIG. 14, and may be connected to a combinationof any of them. Also, each device may be connected directly to thetelephone network ex104, not through the base stations ex107˜ex110 whichare the fixed wireless stations.

The camera ex113 is a device such as a digital video camera capable ofshooting moving pictures. The mobile phone may be any of a mobile phoneof a PDC (Personal Digital Communications) system, a CDMA (Code DivisionMultiple Access) system, a W-CDMA (Wideband-Code Division MultipleAccess) system or a GSM (Global System for Mobile Communications)system, a PHS (Personal Handyphone System) and the like.

Also, a streaming server ex103 is connected to the camera ex113 via thebase station ex109 and the telephone network ex104, which enables livedistribution or the like using the camera ex113 based on the coded datatransmitted from the user. Either the camera ex113 or the server fortransmitting the data may code the data shot by the camera. Also, themoving picture data shot by a camera ex116 may be transmitted to thestreaming server ex103 via the computer ex111. The camera ex116 is adevice such as a digital camera capable of shooting still and movingpictures. In this case, either the camera ex116 or the computer ex111may code the moving picture data. An LSI ex117 included in the computerex111 or the camera ex116 performs coding processing. Note that softwarefor coding and decoding pictures may be integrated into any type of astorage medium (such as a CD-ROM, a flexible disk and a hard disk) thatis a recording medium which can be read by the computer ex111 or thelike. Furthermore, the camera-equipped mobile phone ex115 may transmitthe moving picture data. This moving picture data is the data coded bythe LSI included in the mobile phone ex115.

In this content providing system ex100, contents (such as a music livevideo) shot by users using the camera ex113, the camera ex116 or thelike are coded in the same manner as the above embodiments andtransmitted to the streaming server ex103, while the streaming serverex103 makes stream distribution of the above content data to the clientsat their request. The clients include the computer ex111, the PDA ex112,the camera ex113, the mobile phone ex114 and so on capable of decodingthe above-mentioned coded data. The content providing system ex100 is asystem in which, the clients can thus receive and reproduce the codeddata, and further can receive, decode and reproduce the data in realtime so as to realize personal broadcasting.

When each device in this system performs coding or decoding, the movingpicture coding apparatus or the moving picture decoding apparatus asshown in each of the above-mentioned embodiments may be used.

A mobile phone will be explained as an example thereof.

FIG. 15 is a diagram showing a mobile phone ex115 which uses the movingpicture coding method and the moving picture decoding method asexplained in the above embodiments. The mobile phone ex115 has anantenna ex201 for sending and receiving radio waves between the basestation ex110, a camera unit ex203 such as a CCD camera capable ofshooting video and still pictures, a display unit ex202 such as a liquidcrystal display for displaying the data obtained by decoding video shotby the camera unit ex203, video received by the antenna ex201, or thelike, a main body including a set of operation keys ex204, a voiceoutput unit ex208 such as a speaker for outputting voices, a voice inputunit ex205 such as a microphone for inputting voices, a storage mediumex207 for storing coded or decoded data, such as data of moving or stillpictures shot by the camera, and data of text, moving pictures or stillpictures of received e-mails, and a slot unit ex206 for attaching thestorage medium ex207 into the mobile phone ex115. The storage mediumex207 includes a flash memory element, a kind of EEPROM (ElectricallyErasable and Programmable Read Only Memory) that is an electricallyerasable and rewritable nonvolatile memory, in a plastic case such as anSD card.

Further, the mobile phone ex115 will be explained with reference to FIG.16. In the mobile phone ex115, a main control unit ex311 for overallcontrolling each unit of the display unit ex202 and the main bodyincluding operation keys ex204 is connected to a power supply circuitunit ex310, an operation input control unit ex304, a picture coding unitex312, a camera interface unit ex303, an LCD (Liquid Crystal Display)control unit ex302, a picture decoding unit ex309, amultiplex/demultiplex unit ex308, a record/reproduce unit ex307, a modemcircuit unit ex306 and a voice processing unit ex305 to each other via asynchronous bus ex313.

When a call-end key or a power key is turned ON by a user's operation,the power supply circuit unit ex310 supplies respective units with powerfrom a battery pack so as to activate the camera-equipped digital mobilephone ex115 for a ready state.

In the mobile phone ex115, under the control of the main control unitex311 including a CPU, ROM, RAM and the like, the voice processing unitex305 converts the voice signals received by the voice input unit ex205in conversation mode into digital voice data, the modem circuit unitex306 performs spread spectrum processing of the digital voice data, andthe send/receive circuit unit ex301 performs digital-to-analogconversion and frequency transformation of the data, so as to transmitthe result via the antenna ex201. Also, in the mobile phone ex115, thedata received by the antenna ex201 in conversation mode is amplified andperformed of frequency transformation and analog-to-digital conversion,the modem circuit unit ex306 performs inverse spread spectrum processingof the data, and the voice processing unit ex305 converts it into analogvoice data, so as to output the result via the voice output unit ex208.

Furthermore, when transmitting an e-mail in data communication mode, thetext data of the e-mail inputted by operating the operation keys ex204on the main body is sent out to the main control unit ex311 via theoperation input control unit ex304. In the main control unit ex311,after the modem circuit unit ex306 performs spread spectrum processingof the text data and the send/receive circuit unit ex301 performsdigital-to-analog conversion and frequency transformation of it, theresult is transmitted to the base station ex110 via the antenna ex201.

When picture data is transmitted in data communication mode, the picturedata shot by the camera unit ex203 is provided to the picture codingunit ex312 via the camera interface unit ex303. When the picture data isnot transmitted, the picture data shot by the camera unit ex203 can alsobe displayed directly on the display unit 202 via the camera interfaceunit ex303 and the LCD control unit ex302.

The picture coding unit ex312, including the picture coding apparatusexplained in the present invention, compress and codes the picture dataprovided from the camera unit ex203 by the coding method used for thepicture coding apparatus as shown in the above-mentioned embodiments soas to transform it into coded picture data, and sends it out to themultiplex/demultiplex unit ex308. At this time, the mobile phone ex115sends out the voices received by the voice input unit ex205 duringpicture pickup by the camera unit ex203 to the multiplex/demultiplexunit ex308 as digital voice data via the voice processing unit ex305.

The multiplex/demultiplex unit ex308 multiplexes the coded picture dataprovided from the picture coding unit ex312 and the voice data providedfrom the voice processing unit ex305 by a predetermined method, themodem circuit unit ex306 performs spread spectrum processing of theresulting multiplexed data, and the send/receive circuit unit ex301performs digital-to-analog conversion and frequency transformation ofthe result for transmitting via the antenna ex201.

As for receiving data of a moving picture file which is linked to aWebsite or the like in data communication mode, the modem circuit unitex306 performs inverse spread spectrum processing of the data receivedfrom the base station ex110 via the antenna ex201, and sends out theresulting multiplexed data to the multiplex/demultiplex unit ex308.

In order to decode the multiplexed data received via the antenna ex201,the multiplex/demultiplex unit ex308 demultiplexes the multiplexed datainto a coded bit stream of picture data and a coded bit stream of voicedata, and provides the coded picture data to the picture decoding unitex309 and the voice data to the voice processing unit ex305 respectivelyvia the synchronous bus ex313.

Next, the picture decoding unit ex309, including the picture decodingapparatus explained in the present invention, decodes the coded bitstream of the picture data by the decoding method paired with the codingmethod as shown in the above-mentioned embodiments, so as to generatereproduced moving picture data, and provides this data to the displayunit ex202 via the LCD control unit ex302, and thus moving picture dataincluded in a moving picture file linked to a Website, for instance, isdisplayed. At the same time, the voice processing unit ex305 convertsthe voice data into analog voice data, and provides this data to thevoice output unit ex208, and thus voice data included in a movingpicture file linked to a Website, for instance, is reproduced.

The present invention is not limited to the above-mentioned system.Ground-based or satellite digital broadcasting has been in the newslately, and at least either the picture coding apparatus or the picturedecoding apparatus in the above-mentioned embodiments can beincorporated into such a digital broadcasting system as shown in FIG.17. More specifically, a coded bit stream of video information istransmitted from a broadcast station ex409 to or communicated with abroadcast satellite ex410 via radio waves. Upon receipt of it, thebroadcast satellite ex410 transmits radio waves for broadcasting, a homeantenna ex406 with a satellite broadcast reception function receives theradio waves, and an apparatus such as a television (receiver) ex401 or aset top box (STB) ex407 decodes the coded bit stream for reproduction.The picture decoding apparatus as shown in the above-mentionedembodiments can be implemented in the reproducing apparatus ex403 forreading a coded bit stream recorded on a storage medium ex402 such as aCD and DVD that is a recording medium and decoding it. In this case, thereproduced video signals are displayed on a monitor ex404. It is alsoconceived to implement the picture decoding apparatus in the set top boxex407 connected to a cable ex405 for a cable television or the antennaex406 for satellite and/or ground-based broadcasting so as to reproducethem on a monitor ex408 of the television ex401. The picture decodingapparatus may be incorporated into the television, not in the set topbox. Or, a car ex412 having an antenna ex411 can receive signals fromthe satellite ex410, the base station ex107 or the like for reproducingmoving pictures on a display apparatus such as a car navigation deviceex413 or the like in the car ex412.

Furthermore, the picture coding apparatus as shown in theabove-mentioned embodiments can code picture signals for recording themon a recording medium. As a concrete example, there is a recorder ex420such as a DVD recorder for recording picture signals on a DVD disk ex421and a disk recorder for recording them on a hard disk. They can also berecorded on an SD card ex422. If the recorder ex420 includes the picturedecoding apparatus as shown in the above-mentioned embodiments, thepicture signals recorded on the DVD disk ex421 or the SD card ex422 canbe reproduced for display on the monitor ex408.

Note that as the structure of the car navigation device ex413, thestructure without the camera unit ex203, the camera interface unit ex303and the picture coding unit ex312, out of the units as shown in FIG. 16,is conceivable. The same applies to the computer ex111, the television(receiver) ex401 and others.

In addition, three types of implementations can be conceived for aterminal such as the above-mentioned mobile phone ex114; asending/receiving terminal equipped with both an encoder and a decoder,a sending terminal equipped with an encoder only, and a receivingterminal equipped with a decoder only.

As described above, it is possible to use the moving picture codingmethod or the moving picture decoding method as shown in the aboveembodiments in any of above-mentioned devices and systems, and thus theeffects explained in the above embodiments can be obtained.

In addition, the moving picture coding method and the moving picturedecoding method as shown in the above-mentioned embodiments areeffective when stream data recorded on a storage medium such as a DVD,an SD card and a memory is reproduced at some midpoint of the data.

As is obvious from the above explanation, the moving picture codingmethod according to the present invention is a method for coding amoving picture on a picture by picture basis, the moving picture codingmethod comprising: a determining step for determining, as a referencepicture, one of an intra coded picture and pictures which have beencoded following the intra coded picture, from among a plurality ofpictures stored in a memory; a coding step for reading out thedetermined reference picture from the memory, calculating a differentialpicture signal that is a difference between the read-out referencepicture and a picture to be coded, and coding the calculateddifferential picture signal; and a storing step for decoding the codeddifferential picture signal, adding the decoded differential picturesignal with a picture signal of the reference picture, and storing aresulting picture in the memory.

Accordingly, since coding is not performed with reference to picturespreceding an intra coded picture as reference pictures, reproduction canbe started from the intra coded picture at a midpoint of a coded picturesignal. Also, a coded picture signal, that is a stream of pictures whichcan be reproduced without an error following the intra coded pictureeven if a stream error occurs, can be generated. As mentioned above, thepresent invention has strong advantages in midpoint reproduction whichis important for storage mediums and error resilience for transmissionwhich is important for wireless and cable transmission.

Also, the moving picture decoding method according to the presentinvention is a method for decoding a moving picture on a picture bypicture basis, the moving picture decoding method comprising: a decodingstep for decoding an inputted coded picture signal; a determining stepfor determining, as a reference picture, one of an intra decoded pictureand pictures which have been decoded following the intra decodedpicture, from among a plurality of pictures stored in a memory; and astoring step for reading out the determined reference picture from thememory, adding a picture signal of the read-out reference picture with adecoded differential picture signal of a picture to be decoded, andoutputting a resulting picture to the outside and storing the resultingpicture in the memory.

Accordingly, a coded picture signal, which has been coded with referenceonly to an intra decoded picture or the following pictures as referencepictures, can be properly decoded to obtain a decoded picture signal,and thus reproduction can be started from the intra coded picture atsome midpoint of the coded picture signal. Also, even if an error occursin a stream, the stream can be reproduced without an error following theintra coded picture.

INDUSTRIAL APPLICABILITY

As described above, the moving picture coding method and the movingpicture decoding method according to the present invention are usefulfor starting reproduction from some midpoint of a stream data recordedon a storage medium such as a DVD, an SD card and a memory used for amobile phone, a DVD apparatus, a personal computer or the like.

1. A moving picture coding method for coding a moving picture, wherein in said moving picture coding method, a picture is coded by one of an intra picture coding method and an inter picture coding method, and when a picture is coded by the inter picture coding method, at least one of the following pictures is selected as a reference picture: an intra coded picture which is coded by the intra picture coding method; a picture located before the intra coded picture in display order; and a picture located after the intra coded picture in display order, said moving picture coding method comprises: choosing one of an intra picture coding method and an inter picture coding method as a picture coding method for a current picture; when the inter picture coding method is chosen, coding the current picture by the inter picture coding method; when the intra picture coding method is chosen, specifying the current picture as a specified picture which is used for restricting selection of the reference picture, coding information indicating that the current picture is specified as the specified picture, and coding the current picture by the intra picture coding method, wherein, for a picture to be coded by the inter picture coding method after the specified picture, selecting at least one of the following pictures as the reference picture: the specified picture and a picture which is coded after the specified picture, while prohibiting from selecting, as the reference picture, a picture which has been coded before the specified picture.
 2. The moving picture coding method according to claim 1, further comprising: deleting a picture which has been coded before the specified picture and is stored in the memory so as to prohibit from selecting, as the reference picture, the picture which has been coded before the specified picture.
 3. A moving picture coding apparatus which codes a moving picture, wherein in said moving picture coding apparatus, a picture is coded by one of an intra picture coding unit and an inter picture coding unit, and when a picture is coded by the inter picture coding unit, at least one of the following pictures is selected as a reference picture: an intra coded picture which is coded by the intra picture coding unit; a picture located before the intra coded picture in display order; and a picture located after the intra coded picture in display order, said moving picture coding apparatus comprises: a choosing unit operable to choose one of an intra picture coding unit and an inter picture coding unit as a picture coding unit for coding a current picture; an inter picture coding unit operable to inter-code the current picture when the inter picture coding unit is chosen as the picture coding unit; a specifying unit operable to specify the current picture as a specified picture which is used for restricting selection of the reference picture when the intra picture coding unit is chosen as the picture coding unit. 